Lion’s version of Preview comes with a built-in signature scanner that makes signing documents far simpler. In the Annotations toolbar you now have an option to create a signature from your Mac’s built-in iSight camera. All you need to do is use black ink to sign a piece of white paper, align your signature toward the camera using the onscreen guides, and take a snapshot of the signature. (I haven’t used my real signature here, obviously.)
As it states, I never tried it w/ previous versions so I can’t compare. But having a quick way to sign PDFs will be very helpful!